1. Field of the Invention
The present invention relates to an aqueous dispersion containing polyester resin as a resin component. More particularly, it relates to an aqueous dispersion containing polyester resin suitable for use as a coating for paper, films, steel plates, etc.; various paints for vehicles, cans, etc. including precoat paints; and adhesives.
2. Description of the Prior Art
A large number of organic solvents have been used in the fields of paint; ink; coating; adhesive; and various treatment agents for textiles, paper, and the like. However, increased difficulties have arisen in the use of organic solvents due to a recent trend toward the saving of petroleum resources, prevention of environmental contamination, enforcement of regulations for the discharge of organic solvents such as RACT, restrictions on the use of organic solvents imposed by the amendments of the fire laws, and more rigorous regulations in the concentration of organic solvents in the work environment.
As paint and coatings containing no organic solvents or having low content of organic solvents, there have been proposed paints and coatings of, for example, (i) high solid type, (ii) non-aqueous dispersion type, (iii) aqueous dispersions type, (iv) emulsion type, and (v) solventless type, many of which have already been put into practical use. Among them, those of an aqueous dispersion type are readily handled, thus having widest application, and being in prospect.
Many of the resins contained in the above aqueous dispersion are hydrophobic, which causes the problem of how to make them disperse in water or to maker them water soluble. Further, it is also an important problem that a coating film having high water resistance, corrosion resistance, and weather resistance be obtained by the use of a resin which has been once dispersed or dissolved in water. These problems are true of any resins.
At present, polyester is employed in many cases as the resin contained in the aqueous dispersion mentioned above. As a method for dispersing polyester resin in water or making it water soluble, there are known methods for copolymerizing hydrophilic monomers. For example, there is one known method in which at least one hydrophilic monomer such as diol or dicarboxylic acid containing a metal sulfonate group or polyalkylene glycol is copolymerized with other monomers such as aliphatic dicarboxylic acid. However, all such methods require the use of the hydrophilic monomers mentioned above in large quantities in order to impart excellent solubility or dispersibility in water to the resulting resin, with the result that a coating film formed with a dispersion containing such resin has inferior water resistance and weather resistance.
The use of a combination of a compound containing a metal sulfonate group and polyethylene glycol is disclosed in, for example, Japanese Patent Publication No. 47-40873 and Japanese Laid-Open Patent Publication No. 50-12136. However, since polyester resin containing a large amount of polyethylene glycol has so strong a hydrophilic property that when the resin is used as a binder for printing ink, blocking tends to occur in the printed surfaces due to moisture absorption, making it difficult to use the resin as a binder for printing ink. Similarly, when this polyester resin is used for paint or coating, the resulting coating film absorbs water and moisture, which decreases the adhesion to bases, and causes changes in the appearances of the coating film such as deterioration of gloss, blistering, and a change in the hue, making it impossible to use as a film for the predetermined purpose.
Japanese Patent Publication No. 61-058092 discloses an aqueous dispersion prepared by dispersing polyester resin coating polyalkylene glycol having no aliphatic ether linkages as present in polyethylene glycol, and aromatic dicarboxylic acid containing a metal sulfonate group by use of an amphiphilic solvent. This aqueous dispersion has good dispersibility in water, and a coating film formed with this dispersion has some water resistance. This coating film has enough water resistance so as to serve as a coating film for general use, but does not have high water resistance, or, has interior boiling water resistance. Further, the appearances of the coating film such as the surface smoothness and gloss are not always sufficient. Also, during a coating process, the resin component in paint to be recovered from a coater is partially solidified, and therefore the paint cannot be recovered, or even when recovered, uniform paint cannot be obtained, which causes problems in the working efficiency and productivity.